speedo accuracy and gears
 

I had new tires put on recently. I measured my speedo against the
highway markers today, and it seems it reads a little slow. IOW, my
trip meter was set at the first mile marker and at the completion of
the fourth mile, my trip meter said 3.8. {5% difference}. I probably
should'a tested longer, but....

So my question is, if that's reading slow, would my mph read slower?
So if it says 60, I'm really moving at about 63mph. Anybody know if
this is correct?

I have 3.73 with 30" BFG tires. They actually barely measure a
realistic 29" OD {those liars}. My speedo gear is a 35 tooth. Would
a 34 make it more accurate or maybe it'd throw me off the other way
{show more miles than traveled}?